The invention relates to the field of electronic signal filters and more particularly to the field of active shunt, low pass filters.
There are many applications where it is considered highly desirable to permit low frequency signals including a DC current to flow through the filter and at the same time to provide for rapid attenuation of the signal above a predetermined, or break frequency. For example, in accelerometers it is desirable to pass, with as little attenuation as possible, the DC or low frequency output of the accelerometer while at the same time filtering out higher frequency signals that may be generated by vibration or the like.
In addition to having demanding frequency response characteristics, the use of filters with accelerometers requires that the physical dimensions of the filter be as small as possible since they are quite often used in applications where weight and space are critical such as missile guidance systems. Therefore, in addition to providing the desired operational characteristics, it is considered highly desirable to reduce the actual physical size and weight of the filter components to a minimum. These size and weight considerations tend to rule out purely passive filters due to the fact that they require one or more inductors which, of course, add weight and size along with generally increased cost. Besides being smaller and lighter, active elements generally provide more in the way of design flexibility and better frequency response than filters constructed out of only passive elements, since an active RC network can result in a filter with natural frequencies anywhere in the left half of the complex frequency plane. It is also considered desirable to reduce the number of stages required for a particular order of operation. In many of the prior art active filters, such as Morris U.S. Pat. No. 3,122,714, it is necessary to cascade a number of filter stages to get a higher order of operation.